Adipose tissue and skeletal muscle blood flow during mental stress

SubQ-Sim: A Subcutaneous Physiologically Based Biopharmaceutics Model. Part 1: The Injection and System Parameters

PURPOSE

To construct a detailed mechanistic and physiologically based biopharmaceutics model capable of predicting 1) device-formulation-tissue interaction during the injection process and 2) binding, degradation, local distribution, diffusion, and drug absorption, following subcutaneous injection. This paper is part of a series and focusses on the first aspect.

METHODS

A mathematical model, SubQ-Sim, was developed incorporating the details of the various substructures within the subcutaneous environment together with the calculation of dynamic drug disposition towards the lymph ducts and venous capillaries. Literature was searched to derive key model parameters in healthy and diseased subjects. External factors such as body temperature, exercise, body position, food or stress provide a means to calculate the impact of "life events" on the pharmacokinetics of subcutaneously administered drugs.

RESULTS

The model predicts the tissue backpressure time profile during the injection as a function of injection rate, volume injected, solution viscosity, and interstitial fluid viscosity. The shape of the depot and the concentrations of the formulation and proteins in the depot are described. The model enables prediction of formulation backflow following premature needle removal and the resulting formulation losses. Finally, the effect of disease (type 2 diabetes) or the presence of recombinant human hyaluronidase in the formulation on the injection pressure, are explored.

CONCLUSIONS

This novel model can successfully predict tissue back pressure, depot dimensions, drug and protein concentration and formulation losses due to incorrect injection, which are all important starting conditions for predicting drug absorption from a subcutaneous dose. The next article will describe the absorption model and validation against clinical data.

Cardiopulmonary values and organ blood flows before and during heat stress: data in nine subjects at rest in the upright position

Physiological changes associated with thermoregulation can influence the kinetics of chemicals in the human body, such as alveolar ventilation (VA) and redistribution of blood flow to organs. In this study, the influence of heat stress on various physiological parameters was evaluated in nine male volunteers during sessions of exposure to wet blub globe temperatures (WBGT) of 21, 25 and 30°C for four hours. Skin and core temperatures and more than twenty cardiopulmonary parameters were measured. Liver, kidneys, brain, skin and muscles blood flows were also determined based on published measurements. Results show that most subjects (8 out of 9) have been affected by the inhalation of hot and dry air at the WBGT of 30°C. High respiratory rates, superficial tidal volumes and low VA values were notably observed. The skin blood flow has increased by 2.16-fold, whereas the renal blood flow and liver blood flow have decreased by about by 11 and 18% respectively. A complete set of key cardiopulmonary parameters in healthy male adults before and during heat stress was generated for use in PBPK modeling. A toxicokinetic studies are ongoing to evaluate the impact of heat stress on the absorption, biotransformation and excretion rates of volatile xenobiotics.

Muscle microvasculature's structural and functional specializations facilitate muscle metabolism

We review the evolving findings from studies that examine the relationship between the structural and functional properties of skeletal muscle's vasculature and muscle metabolism. Unique aspects of the organization of the muscle microvasculature are highlighted. We discuss the role of vasomotion at the microscopic level and of flowmotion at the tissue level as modulators of perfusion distribution in muscle. We then consider in some detail how insulin and exercise each modulate muscle perfusion at both the microvascular and whole tissue level. The central role of the vascular endothelial cell in modulating both perfusion and transendothelial insulin and nutrient transport is also reviewed. The relationship between muscle metabolic insulin resistance and the vascular action of insulin in muscle continues to indicate an important role for the microvasculature as a target for insulin action and that impairing insulin's microvascular action significantly affects body glucose metabolism.

Evidence for centrally induced cholinergic vasodilatation in skeletal muscle during voluntary one-legged cycling and motor imagery in humans

We have recently reported that central command contributes to increased blood flow in both noncontracting and contracting vastus lateralis (VL) muscles at the early period of voluntary one-legged cycling. The purpose of this study was to examine whether sympathetic cholinergic vasodilatation mediates the increases in blood flows of both muscles during one-legged exercise. Following intravenous administration of atropine (10 μg/kg), eight subjects performed voluntary 1-min one-legged cycling (at 35% of maximal voluntary effort) and mental imagery of the exercise. The relative concentrations of oxygenated- and deoxygenated-hemoglobin (Oxy- and Deoxy-Hb) in the bilateral VL were measured as an index of muscle tissue blood flow with near-infrared spectroscopy (NIRS). The Oxy-Hb in both noncontracting and contracting VL increased at the early period of one-legged cycling, whereas the Deoxy-Hb did not alter at that period. Atropine blunted (P < 0.05) the Oxy-Hb responses of both VL muscles but did not affect the Deoxy-Hb responses. The time course and magnitude of the atropine-sensitive component in the Oxy-Hb response were quite similar between the noncontracting and contracting VL muscles. With no changes in the Deoxy-Hb and hemodynamics, imagery of one-legged cycling induced the bilateral increases in the Oxy-Hb, which were completely abolished by atropine. In contrast, imagery of a circle (with no relation to exercise) did not alter the NIRS signals, irrespective of the presence or absence of atropine. It is concluded that central command evokes cholinergic vasodilatation equally in bilateral VL muscles during voluntary one-legged cycling and motor imagery.

Evaluation of specific metabolic rates of major organs and tissues: comparison between nonobese and obese women

Elia (1992) identified the specific resting metabolic rates (K(i)) of major organs and tissues in young adults with normal weight: 200 for liver, 240 for brain, 440 for heart and kidneys, 13 for skeletal muscle, 4.5 for adipose tissue and 12 for residual mass (all units in kcal/kg per day). The aim of the present study was to assess the applicability of Elia's K(i) values for obese adults. A sample of young women (n = 80) was divided into two groups, nonobese (BMI <29.9 kg/m(2)) and obese (BMI 30.0-43.2 kg/m(2)). This study was based on the mechanistic model: REE = σ (K(i) × T(i)), where REE is whole-body resting energy expenditure measured by indirect calorimetry and T(i) is the mass of individual organs and tissues measured by magnetic resonance imaging. For each organ/tissue, the corresponding Elia's K(i) value was analyzed respectively for nonobese and obese groups by using stepwise univariate regression analysis. Elia's K(i) values were within the range of 95% confidence intervals (CIs) in the nonobese group. However, Elia's K(i) values were outside the right boundaries of 95% CIs in the obese group and a corresponding obesity-adjusted coefficient was calculated as 0.98, indicating that Elia's values overestimate K(i) by 2.0% in obese adults. Obesity-adjusted K(i) values were 196 for liver, 235 for brain, 431 for heart and kidneys, 12.7 for skeletal muscle, 4.4 for adipose tissue, and 11.8 for residual mass. In conclusion, although Elia's K(i) values were validated in nonobese women, obesity-adjustments are appropriate for application in obese women.

Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure

BACKGROUND

The specific resting metabolic rates (K(i); in kcal · kg(-1 )· d(-1)) of major organs and tissues in adults were suggested by Elia (in Energy metabolism: tissue determinants and cellular corollaries. New York, NY: Raven Press, 1992) to be as follows: 200 for liver, 240 for brain, 440 for heart and kidneys, 13 for skeletal muscle, 4.5 for adipose tissue, and 12 for residual organs and tissues. However, Elia's K(i) values have never been fully evaluated.

OBJECTIVES

The objectives of the present study were to evaluate the applicability of Elia's K(i) values across adulthood and to explore the potential influence of age on the K(i) values.

DESIGN

A new approach was developed to evaluate the K(i) values of major organs and tissues on the basis of a mechanistic model: REE = Σ(K(i) × T(i)), where REE is whole-body resting energy expenditure measured by indirect calorimetry, and T(i) is the mass of individual organs and tissues measured by magnetic resonance imaging. With measured REE and T(i), marginal 95% CIs for K(i) values were calculated by stepwise univariate regression analysis. An existing database of nonobese, healthy adults [n = 131; body mass index (in kg/m²) <30] was divided into 3 age groups: 21-30 y (young, n = 43), 31-50 y (middle-age, n = 51), and > 50 y (n = 37).

RESULTS

Elia's K(i) values were within the range of 95% CIs in the young and middle-age groups. However, Elia's K(i) values were outside the right boundaries of 95% CIs in the >50-y group, which indicated that Elia's study overestimated K(i) values by 3% in this group. Age-adjusted K(i) values for adults aged >50 y were 194 for liver, 233 for brain, 426 for heart and kidneys, 12.6 for skeletal muscle, 4.4 for adipose tissue, and 11.6 for residuals.

CONCLUSION

The general applicability of Elia's K(i) values was validated across adulthood, although age adjustment is appropriate for specific applications.

Emerging role of adipose tissue hypoxia in obesity and insulin resistance

Recent studies consistently support a hypoxia response in the adipose tissue in obese animals. The observations have led to the formation of an exciting concept, adipose tissue hypoxia (ATH), in the understanding of major disorders associated with obesity. ATH may provide cellular mechanisms for chronic inflammation, macrophage infiltration, adiponectin reduction, leptin elevation, adipocyte death, endoplasmic reticulum stress and mitochondrial dysfunction in white adipose tissue in obesity. The concept suggests that inhibition of adipogenesis and triglyceride synthesis by hypoxia may be a new mechanism for elevated free fatty acids in the circulation in obesity. ATH may represent a unified cellular mechanism for a variety of metabolic disorders and insulin resistance in patients with metabolic syndrome. It suggests a new mechanism of pathogenesis of insulin resistance and inflammation in obstructive sleep apnea. In addition, it may help us to understand the beneficial effects of caloric restriction, physical exercise and angiotensin II inhibitors in the improvement of insulin sensitivity. In this review article, literatures are reviewed to summarize the evidence and possible cellular mechanisms of ATH. The directions and road blocks in the future studies are analyzed.

Marked increase in white adipose tissue blood perfusion in the type 2 diabetic GK rat

The aim of the present study was to evaluate and cor-relate islet to brown and white adipose tissue (WAT) blood perfusion in one obese rat and one nonobese rat with type 2 diabetes (obese Zucker [OZ] and GK rats, respectively). We measured blood perfusion with a microsphere technique in anesthetized animals and subsequently estimated the blood flow to seven different WAT depots and brown adipose tissue, in addition to the whole pancreas and pancreatic islets. Both GK and OZ rats had higher islet blood perfusion than their respective control strains. Adipose tissue blood flow (ATBF) was similar to or lower than that of controls in the normoglycemic OZ rats. GK rats, however, had 5-10 times higher blood perfusion than control Wistar rats in most WAT depots. Vascular density and macrophage numbers in WAT did not differ between the different strains. The discrepancy in ATBF between the obese-normoglycemic and type 2 diabetic rats opens the intriguing possibility that changes in this blood perfusion may influence and/or modulate the beta-cell dysfunction in type 2 diabetes.

Impaired postprandial adipose tissue blood flow response is related to aspects of insulin sensitivity

Obesity has been associated with dysfunctional postprandial adipose tissue blood flow (ATBF), but it has also been recognized that the interindividual response is highly variable. The present work aimed at characterizing this variability. Fifteen subjects were given 75 g oral glucose, and abdominal subcutaneous ATBF was monitored by the (133)Xe washout method. Determinants of insulin sensitivity based on nonesterified fatty acid (NEFA) suppression after oral glucose administration [ISI(NEFA)] were higher in the top tertile ATBF response group (1.29 +/- 0.09 vs. 0.90 +/- 0.08 in the lower tertiles, P = 0.01). ISI(NEFA) was related to ATBF response (r(s) = 0.73, P < 0.002) as well as insulin sensitivity based on postprandial glycemia [ISI(gly), r(s) = 0.58, P < 0.05], whereas the homeostasis model assessment (HOMA) index (r(s) = -0.39, P = 0.16) was not. The relationship between increase in ATBF and ISI(NEFA) was independent of BMI (P = 0.015) in multivariate analysis. Subjects with a high ATBF response had significantly higher increase of plasma norepinephrine (P < 0.05), indicating a link between postprandial insulinemia, sympathetic activation, and ATBF response. There is a close relationship between insulin sensitivity and the regulation of postprandial ATBF, independent of adiposity. Impaired regulation of ATBF seems to be another facet of the insulin resistance syndrome.

Effects of mental stress on insulin-mediated glucose metabolism and energy expenditure in lean and obese women

The effects of the sympathetic activation elicited by a mental stress on insulin sensitivity and energy expenditure (VO(2)) were studied in 11 lean and 8 obese women during a hyperinsulinemic-euglycemic clamp. Six lean women were restudied under nonselective beta-adrenergic blockade with propranolol to determine the role of beta-adrenoceptors in the metabolic response to mental stress. In lean women, mental stress increased VO(2) by 20%, whole body glucose utilization ([6,6-(2)H(2)]glucose) by 34%, and cardiac index (thoracic bioimpedance) by 25%, whereas systemic vascular resistance decreased by 24%. In obese women, mental stress increased energy expenditure as in lean subjects, but it neither stimulated glucose uptake nor decreased systemic vascular resistance. In the six lean women who were restudied under propranolol, the rise in VO(2), glucose uptake, and cardiac output and the decrease in systemic vascular resistance during mental stress were all abolished. It is concluded that 1) in lean subjects, mental stress stimulates glucose uptake and energy expenditure and produces vasodilation; activation of beta-adrenoceptors is involved in these responses; and 2) in obese patients, the effects of mental stress on glucose uptake and systemic vascular resistance, but not on energy expenditure, are blunted.

Effect of exercise training on in vivo lipolysis in intra-abdominal adipose tissue in rats

Intra-abdominal obesity is associated with cardiovascular disease and non-insulin-dependent diabetes mellitus, and physical training has been suggested to alleviate these conditions. We compared epinephrine-stimulated lipolysis in vivo in three intra-abdominal adipose tissues (ATs: retroperitoneal, parametrial, and mesenteric) and in subcutaneous AT, and we also studied the effect of physical training. Moreover, we studied the effect of physical training on epinephrine-stimulated lipolysis in muscle in vivo. Female rats were either swim trained (15 wk, n = 8) or sedentary (n = 7). Under anesthesia, a two-stage intravenous epinephrine infusion (60 min of 80 and 200 ng. kg(-1). min(-1), respectively) was carried out, and local interstitial glycerol concentration was measured by the microdialysis technique. Blood flow was measured by microspheres. Training increased blood flow in all ATs [on average: 73 +/- 12 (trained) vs. 14 +/- 4 (sedentary) ml. 100 g(-1). min(-1), P < 0. 05]; nevertheless, epinephrine-stimulated interstitial glycerol concentrations were increased or unchanged. Interstitial glycerol concentration was higher in intra-abdominal than in subcutaneous AT in both trained and sedentary rats. In skeletal muscle, interstitial glycerol concentration and blood flow did not differ between trained and sedentary rats. In conclusion, in vivo lipolysis is higher both in the basal state and during epinephrine-stimulation in intra-abdominal than in subcutaneous AT, and training may be beneficial in alleviating intra-abdominal obesity by enhancing lipolysis in intra-abdominal fat depots.

Viability of fat obtained by syringe suction lipectomy: effects of local anesthesia with lidocaine

The results of transplantation of free autologous fat obtained by blunt syringe suction lipectomy are unpredictable. We examined if adipose tissue viability is compromised by using syringe suction lipectomy and by infiltration of the tissue with local anesthetics. As reference, we used adipose tissue samples excised during elective surgery. Fat obtained intraoperatively and by lipectomy was digested with collagenase to isolate adipocytes. The mechanical damage associated with sample handling and cell isolation in both procedures was similar and did not exceed 6% of the total cell mass. In addition, cells isolated from intraoperative and lipectomy samples did not differ functionally, responded similarly to insulin stimulation of glucose transport and epinephrine-stimulated lipolysis, and retained the same growth pattern in culture. Since during fat transplantation the graft is exposed to local anesthetics at both the donor and the recipient sites, we reexamined adipocyte function in the presence of lidocaine. Lidocaine potently inhibited glucose transport and lipolysis in adipocytes and their growth in culture. That effect, however, persisted only as long as lidocaine was present; after washing, the cells were able to fully regain their function and growth regardless of whether the exposure was as short as 30 minutes or as long as 10 days. These results indicate that adipose tissue obtained by syringe lipectomy consists of fully viable and functional adipocytes, but local anesthetics may halt their metabolism and growth.

Demonstration of an in vivo functional beta 3-adrenoceptor in man

Although it is well established in several mammalian species that beta 3-adrenoceptors play a major role in regulating lipolysis and thermogenesis in adipose tissue, the functional existence and role of this receptor subtype in man has been controversial. We investigated whether the beta 3-adrenoceptor functionally co-exists with beta 1- and beta 2-adrenoceptors in vivo in human adipose tissue. Subcutaneous abdominal adipose tissue of healthy non-obese subjects was microdialyzed with equimolar concentrations of dobutamine (selective beta 1-adrenoceptor agonist), terbutaline (selective beta 2-adrenoceptor agonist), or CGP 12177 (selective beta 3-adrenoceptor agonist). All three agents caused a rapid, sustained, concentration-dependent and significant elevation of the glycerol level in the microdialysate (lipolysis index). However, only terbutaline stimulated the nutritive blood flow in adipose tissue, as measured by an ethanol escape technique. Dobutamine and CGP 12177 was equally effective in elevating the glycerol level (maximum effect 150% above baseline). Terbutaline was significantly more effective than the other two beta-agonists (maximum effect 200% above baseline). When adipose tissue was pretreated with the beta 1/beta 2-selective adrenoceptor blocker propranolol the glycerol increasing effect of dobutamine or terbutaline was inhibited by 80-85% but the glycerol response to CGP 12177 was not influenced. It is concluded that a functional beta 3-adrenoceptor is present in vivo in man. It co-exists with beta 1- and beta 2-adrenoceptors in adipose tissue and may therefore play a role in lipolysis regulation. It appears, however, that the beta 2-adrenoceptor is the most important beta-adrenoceptor subtype for the mobilization of lipids from abdominal subcutaneous adipose tissue because of its concomitant stimulatory effect on lipolysis and blood flow.

Lipolytic catecholamine resistance due to decreased beta 2-adrenoceptor expression in fat cells

The existence of lipolytic beta-adrenoceptor (BAR) resistance was investigated in vivo and in isolated abdominal subcutaneous adipocytes in 65 healthy and drug-free subjects. The concentration of isoprenaline (nonselective BAR agonist) causing half-maximum lipolysis effect (ED50) varied bimodally and 10(6)-fold between individuals but was almost constant in the same subject when measured two times at rest or before and 30 min after exercise. The subjects were categorized as having either high or low isoprenaline sensitivity. The former group had a 50% reduced in vivo lipolytic response to exercise and mental stress, despite a 50% increased plasma noradrenaline response (P < 0.01) and a 350% increased plasma adrenaline response (P < 0.02). In fat cells the lipolytic ED50 values for noradrenaline and terbutaline (BAR2 agonist) were 10 times lower (P < 0.001) in low-sensitive subjects, but the maximum lipolytic actions of these agents (and of isoprenaline) were similar in both groups. The action on lipolysis of dobutamine (BAR1 agonist), forskolin (stimulating adenylate cyclase), dibutyryl cyclic AMP (activating protein kinase), clonidine (alpha 2-adrenergic agonist), or phenyl isopropyladenosine (adenosine receptor agonist) were almost identical in high- and low-sensitivity subjects. ED50 for isoprenaline correlated with ED50 for terbutaline (r = 0.75), but not with ED50 for dobutamine. In high-sensitivity subjects the number of BAR2 was almost three-fold increased (P < 0.002) and the steady-state adipocyte mRNA level for BAR2 was sixfold increased (P < 0.005). BAR2 affinity as well as BAR1 number, affinity and mRNA expression were similar in both groups. In 11 cholecystectomy patients (otherwise healthy) lipolytic ED50 for beta agonists correlated in omental and subcutaneous fat cells (r = 0.85 for isoprenaline; r = 0.95 for terbutaline). In conclusion, lipolytic resistance to catecholamines is present in vivo in apparently healthy subjects due to reduced expression of BAR2 in adipocytes.